There is no way to use the keyboard to dismiss some of the popups opened by clicking plasmoids (for example, device notifier, system tray, user switcher, networks, media player, battery, kdeconnect). This feature is necessary, as sometimes popups are opened without mouse interaction and steal focus (for eg., when a new removable device is plugged in). 

PS: the escape key works for some of the plasmoids (for eg, k-launcher, kmix, klipper), so I am not sure if this is a bug in plasmashell or the individual plasmoids.